The FBI is actively investigating a report that three terrorists entered the U.S., in August with the intent to launch an attack on the anniversary of 9-11. A credible threat was pinpointed Wednesday night.

Security officials have said they are searching for two large rental trucks that may be loaded with explosive material and bound for N.Y. or Wash. D.C. Railways and ports are also under surveillance. The vehicles have tentatively been identified as large Penske or Ryder trucks, according to sources in law enforcement.

"As we know from the intelligence gathered from the Osama Bin Laden raid, al Qaeda has shown an interest in important dates and anniversaries, such as 9/11," Chandler said. "In this instance, it is accurate that there is specific, credible, but unconfirmed threat information."

White House Senior Advisor Valerie Jarrett said that President Obama is fully aware of the threat and he is "updated hourly on the investigation." She added that he was fully briefed Wednesday and Thursday on the operations.

Security sources confirmed Thursday night that three suspects are being sought and there is intense scrutiny at railways, subways, ports and high profile targets in the nation's capitol and New York City; however the reports are not confirmed as of Thursday night.

"As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days, Chandler said."Sometimes this reporting is credible and warrants intense focus, other times it lacks credibility and is highly unlikely to be reflective of real plots underway.”

National security experts indicated that "chatter" has markedly increased in known terrorist circles. The information was detected by U.S. and other security surveillance programs in the Middle East and Asia. When "chatter" escalates it may be indicative of a pending terrorist attack, similar to what happened in the days before 9/11., according to officials with knowledge of the information.

Additionally, information mined during the operation to kill Bin Laden indicated that attack plans for 9/11, included crowded areas and railways in highly populated areas.

Homeland Security is asking the American People to report any suspicious activity in the area by calling 9-1-1 or their nearest FBI field office.

“We take all threat reporting seriously,” Chandler said. “We have taken, and will continue to take all steps necessary to mitigate any threats that arise. We continue to ask the American people to remain vigilant as we head into the weekend.”